1. Field of the Invention
The invention relates to a method of mechanochemical etching of the side wall of semiconductor plates whose main faces are substantially circular, and to an apparatus for carrying out the method.
It is known that, in order to obtain certain semiconductor devices, notably diodes and in particular solar cells, a p-n junction is formed by diffusing n-type impurities from all the walls of a p-type plate serving as a substrate, and also from the side walls.
The following operation then consists of making contacts on the P and N type regions. The contact on the N region is made in a simple and rapid manner but when making contacts on the P region numerous difficulties occur. Until now the most certain solution consists in the diffusion of impurities of aluminium from a region of the surface of the N layer into the P region to obtain a contact area at the surface.
No matter what method is adopted, it is advisable, however, to eliminate the diffused N type region from the side walls so as to prevent possible short-circuits.
2. Description of the Invention
Until now, removal of the N layer has been performed mechanically and has been manually controlled, the most simple method consisting of grinding the side wall by means of an abrasive until the original P region appears. The success of this method depends upon the ability of the operator so that the results obtained are generally not reproducible. In addition, the grinding thickness is not uniform and facets and ridges may be formed which are fragile.
In addition, grinding presents the drawback of creating a perturbation of the crystal lattice in the proximity of the ground region, the result of said perturbation being a diminishing of the photoelectronic efficiency of the ground semiconductor plate.
These drawbacks are accentuated in the case of circular semiconductor plates generally used for the manufacture of solar cells.
Another recommended method consists of dipping the semiconductor plates in a bath containing an etching solution, after first having covered the two main faces of the plates with an etchant-resistant layer, for example picein. Such a method requires additional steps and handling, and increased care. Moreover, the results are not always effective, nor reproducible. Actually, the protective layer does not necessarily have a uniform thickness nor a complete adherence to the surface to be protected; as a result infiltration of etching solution may occur and produce defects in the active regions of the already manufactured plate.
None of the known methods, particularly those described above permits automation of the operation so as to reduce its cost.